Eight times the mean motion of Venus is so nearly equal to thirteen times that of the earth that the difference amounts to only the 2.
Instead of the period it is common in astronomical practice to use the mean angular speed, called the mean motion of the body.
It follows that putting n for the mean motion and T for the period of revolution we shall have in degrees nT=3600.
In these cases therefore the mean distance and mean motion are regarded as different elements, and the whole number of the latter is seven.
Putting a for the mean distance of the earth from the sun, and n for its mean motion in one second, we use the fundamental equation a3 n2 = Mo-1-M', Mo being the sun's mass, and M' the combined masses of the earth and moon, which are, however, too small to affect the result.
